Nowadays paper made of natural pulp is replaced with polyene synthetic paper of which the basic substance layer (intermediate layer) is the biaxial oriented polypropylene (BOPP) whereon the back surface is laminated with the uniaxial oriented polypropylene (UOPP) of 8-65% weight inorganic fine powder as the paper surface layer (see Japanese Patents 46-40794, 56-141339 and 56-118437 granted to TOKO Kaichaku).
The core of the foregoing synthetic paper in general is the inorganic fine powder and there are fine empty holes around the paper of which the surface has a plurality of stripe-shaped cracks. In order to enhance the intaglio printing, the said surface is coated with the aqueous solution of acrylic acid series polymer and polyimide and then dried for use (see Japanese Patents 50-10624 and 50-161478 granted to Toko Kaichaku.)
In addition, Toko Kaihai's Japanese Patent 3-87255 disclosed using the biaxial oriented film of resin with high crystallizability polypropylene of 35-95% weight and talcum powder and/or mica powder of 65-5% weight over 98% isotacticity as the basic substance layer (intermediate layer) of which at least one surface is laminated with a paper layer formed by the uniaxial oriented film of resin with high crystallizability polypropylene of 20-90% weight and inorganic fine powder of 80-10% weight over 98% isotacticity and finally a multi-layer resin synthetic paper with over 90% opacity is formed.
Such a synthetic paper has the advantage of rigidity, mechnical strength and printing head contact but leaves the opacity, low density and imitation paper surface friction coefficient to be desired, and so far as the resin composite and process thereof are conerned, the produced synthetic paper has to be coated with the aqueous solution of acrylic acid series polymer, etc. to increase the post-treatment of printing, so the opacity, low desnity and imitation paper surface friction coefficient of the said synthetic paper are poor, that is why it is unsuitable for cultural paper. Furthermore, the paper surface layer of said synthetic paper is finished through the upper and lower layer-increasing filming by two extruders between longitudinal and lateral orientation devices. The paper surface layer of finished filming can only be laterally oriented (uniaxially oriented), the surface strength thereof is lower, and the calcium carbonate applied to the said surface is porous and tends to be penetrated by the solvent to deform the said surface and to cause dimensional change, so it is less suitable for printing, and the calcium carbonate on the said surface will come off to trouble the printing operation of printing machine. In addition, the configuration of calcium carbonate is more irregular and causes a coarse surface with a sense of poor quality.
In order to solve the foregoing problem, the present invention adopts a synthetic paper with 3-layer structure, namely, paper surface layer/intermediate layer/paper surface layer, wherein the PP resin composite of intermediate layer is extruded by a primary extruder, and the paper surface layer is the resin composite extruded by two secondary extruders, and all these composites are extruded as an extrudate co-flow by same one T-type die. Therefore, the said synthetic paper with 3-layer structure is made through cooling/shaping, biaxial orientation, corona discharge treatment and laminating/rolling depending on the required thickness (over 100.mu.). It is different from the 2-layer synthetic paper according to the foregoing Japanese Patene 3-87255 wherein the paper surface is only uniaxially oriented and then layer-increased through filming.